Integrated Gate Bipolar Transistor (IGBT) technology reached new highs with its deployment in voltage source converters (VSC) for power system applications such as HVDC transmission and power quality management.
Today, the IGBT is the preferred choice for such applications because of the following features:                Low-power control, since it is a MOS-controlled device, e.g. advantageous when operating at very high voltage levels (several 100 kV)        Transistor action, which allows precise controlling of the device in a manner that is not possible with latching alternatives (for instance, the converter can be turned off even in short circuit conditions)        High switching speed, thus making high switching frequency feasibleWhile well suited electrically, the IGBT did not reach its current status in such high power, high profile applications until a new press pack technology was introduced. Some of the key packaging aspects were redesigned thus allowing to advance from traditional thyristor-based line-commutated converter technology to IGBT-based voltage-source technology.        
The competing IGBT press pack packages available today are adapted from traditional thyristor ,hockey puck’ packages. This rigid pressure contact technology is not optimized to protect sensitive microstructures on the surface of IGBT chips. As a consequence users are required to provide near-perfect cooler surfaces and handle such devices with a great deal of care during assembly. The issue is further aggravated when the module size is increased for higher current ratings. There is a significant cost impact on system production cost as a result of these shortcomings.
Converters ranging in power from a few to several hundreds of MW utilize considerable numbers of semiconductor devices. As converter voltages reach tens of hundreds of kV, series connection of a large number of semiconductor devices is essential. Most suited for series connection is the stacking of devices on top of each other, well known from thyristors. An IGBT module suited for such an application has to fulfill the following mechanical requirements:                In order to provide a satisfactory mechanical stability during transport and operation of an assembled stack, which can be several meters long, a high clamping force of up to 100 kN is mandatory.        To minimize system and assembly cost, high tolerance to pressure non-uniformity is required.        
A new pressure-contact technology was introduced in U.S. Pat. No. 5,705,853. The direct pressure on the chip is decoupled from the external clamping force by using a flexible emitter contact (individual press-pin) in combination with a stiff housing as shown schematically in FIG. 1.
The contact partners (individual press pins) 6 on the chips 1 are flexible and upon clamping, they are compressed until top plate 3 and base plate 2 touch the stiff housing elements 4. When the external force is further increased the pressure on the chips 1 will remain stable, whereas the housing 4 will take the additional force. Top plate 3 has to be thick enough to allow vertical movement of the rod that guides the flexible press pin contact elements and not to bend under the pressure applied by these contact elements 6. The individual press pins 6 allow a homogenous pressure distribution even if a large number of chips are arranged inside one module.
The significant advantage of this concept is that it is much less sensitive to pressure inhomogenities compared to traditional ,hockey-puck’ designs with stiff copper polepieces and that it allows very high mounting force as well as much wider mechanical tolerances. This results in an increased mechanical reliability at reduced costs.
In HVDC systems operating at high line voltages, numerous devices are normally connected in series. One large VSC based HVDC station, which handles several hundred MW is likely equipped with more than thousand IGBT modules in total. By adding extra devices in the stack of series connected devices, redundancy can be built into the system. This enables operating the system even if some of the individual semiconductor devices fail, securing a high availability of the system and minimizing the need for periodic maintenance.
Since the devices are operated in series connection, it is a prerequisite for such redundancy that the devices fail in a controlled manner, forming a short circuit with sufficiently low resistance to be able to conduct the total current in the system. The devices are not allowed to fail open circuited and thereby cause disruption of the load current. The failed components, working in the Short Circuit Failure Mode (SCFM) are replaced later during scheduled maintenance.
In order to increase SCFM-reliability a new packaging technology has been developed. EP 0 989 611 describes a semiconductor module with long-term stable SCFM, even at low currents. The silicon of the semiconductor chip is metallurgically alloyed with an optimized contact partner. A low melting compound is formed leading to a highly conductive path through the chip. The alloying of the chip occurs immediately after the failure, when a high current strike causes the metallurgically optimized material, which is pressed onto the chip, to melt and react with the underlying silicon. The result is a reliable SCFM performance during, after-life’ operation in the system.
It would be economically advantageous to standardize stack design for converters of various current ratings. It is therefore preferred that the overall package is fixed for a range of IGBT current ratings, without significant increase in the cost of devices with lower current ratings.